Re: [PSUBS-MAILIST] Ribs

["Sean T. Stevenson" <cast55@telus.net>]

According to Section 6 of the ABS Rules, cylindrical hulls are assessed based 
on the following criteria:

1)  inter-stiffener strength
2)  longitudinal stress at frame
3)  overall buckling strength
4)  stiffeners, with particular checks for:
	a)  stress limits
	b)  stiffener tripping
	c)  local buckling
	d)  inertia requirements
5)  exostructure loads

So, changing your stiffener geometry will affect the result of one or more of 
these failure modes, but the maximum allowable working pressure will not 
change unless your geometry change affects the particular stress value that was 
the limiting factor.  What this effectively means is that a geometry change 
which does not change the maximum allowable working pressure implies that some 
aspect of the geometry was overdesigned.  For the most efficient design 
possible, the allowable working pressures for each possible failure mode 
should be similar, although it is desireable to have one which is somewhat 
dominant, so that the actual failure mode of your hull is deterministic.

The number of lobes expected at failure is not the number of ribs.  Rather, it 
is the number of distinct radial sectors present at shell collapse, or if you 
like, the number of "folds" that occur in the radial direction when the shell 
collapses.  Similar to an inter-stiffener strength failure which would produce 
a characteristic "accordion" shape between stiffeners, a shell collapse in 
overall buckling folds in the other direction - imagine that instead of a 
cylinder, which has a circle as its cross-sectional shape, you had a 
triangular cross-section - at collapse, this would force three lobes at 
failure.  Similarly for a hexagonal shape, you would have six lobes.  With a 
perfect circle, it is not so easy to predict what shape it will assume at 
collapse.  Have a look at Figure 4 in Section 6 of the ABS Rules - this gives 
you some idea of what to expect, but in fact you should repeat your stress 
calculation for a number of values of n (number of overall instability lobes), 
and use the one which gives the lowest maximum allowable pressure.

-Sean




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